HMF glasses, both vitreous and crystalline, possess desirable optical properties for potential use as mid-IR optical fibers, including a broad transmittance range from the mid-IR (approximately 7 microns) to near-UV (approximately 0.3 microns), low absorption and scatter losses, and high tensile strength.
During the past several years a number of artisans have demonstrated the feasibility of using heavy metal fluoride (HMF) glasses for a variety of ultralow-loss fiber optic waveguides and mid-IR optical fiber applications. Included have been ultralong repeaterless links, nuclear radiation resistant links, high-capacity wavelength multiplexed fiber optic systems, mid-IR power delivery fibers, and long-length fiber optic sensor systems.
Despite these impressive results, HMF glasses exhibit problems which tend to limit their further development and future application. Specifically, HMF glasses degrade rapidly when exposed to high humidity, are chemically sensitive, and have low softening temperatures, typically 200 degrees C.
Several artisans have investigated sealing the fiber surface with a hermetic layer to prevent ambient moisture from contacting the glass and to thereby prevent subsequent stress corrosion. While some progress has been made in understanding and alleviating these problems, no solution to these difficulties is yet available. This is indeed unfortunate considering that much of the underlying technology for HMF glass and optical fibers development is established and ready for implementation. Prior studies have shown that the solubility of typical fluorozirconates, a category of heavy metal fluoride (HMF) glasses, is many orders of magnitude greater than that of silicates. Additionally, prior measurements of fiber strength reveal that HMF optical fibers degrade rapidly when exposed to high humidity. Finally, it is generally known that conventional protective Teflon coatings do not serve as effective barriers against the degradation. Thus, despite their promising optical characteristics, many fluoride glass compositions are relatively soft and hygroscopic, thereby preventing their practical use.
Compounding the above problems, fluoride glass materials have low softening temperatures, typically 200 degrees C. This precludes application of standard thin film deposition techniques which require a substrate temperature in excess of 250-300 degrees C. In conventional deposition schemes this elevated substrate temperature is required to produce thin films which are durable, non-porous, and have good substrate adhesion; otherwise, the film does not provide a good hermetic coating.
An additional problem exists when one considers coating HMF glasses in fiber form, as they are being pulled. Typically, the rate that fiber is drawn exceeds 3 meters/minute. To form a hermetic coating on the fiber, a minimum thickness of 1000 Angstroms is required. In the case of a practical coating apparatus operating on the fly, a fiber coating region approximately 25 centimeters in length should be constructed. During a 5-second time period in which the fiber passes through the 25-centimeter long region, it must be coated with a film 1000 Angstroms in thickness. This dictates a deposition rate of approximately 200 Angstroms/second, which is approximately 60 times that of conventional deposition techniques. This increased deposition rate, combined with low (200 degrees C.) substrate temperature, can have a profound deleterious influence on film morphology and hence on film hermeticity and durability. A coating apparatus which would scale to several meters in length would allow a proportionate reduction in the film deposition rate from that of a 25-centimeter length.
HMF optical fibers, loaded rapidly or forced to support a given load for a short time, are relatively strong, whereas those fibers are relatively weak if loaded slowly or forced to support a load for a longer time. Furthermore, the strength of fibers decreases as temperature increases and the relative humidity of the surrounding environment increases. Fibers have been found to be weakest when immersed in water. This susceptibility to attack by moisture indicates that hermetic coatings are required to protect HMF glasses. Moreover, because HMF glasses are chemically sensitive, an additional requirement is that optical coatings must be applied in a non-damaging way.
There are a number of known ways to guard against HMF optical fiber fatigue. However, the most attractive alternative still remains a hermetic coating to protect the fiber. Some known artisans coated the glass fibers with a UV-curable epoxy acrylate to increase the long-term strength of the glass fibers. Others have utilized metal to protect the surface of the fibers. Yet others have deposited diamond-like carbon (DLC) to provide a number of properties that could result in improved optical elements such as windows and mirrors for high powered lasers. A polycrystalline aluminum applied by freeze-coating suffers from cyclic fatigue and subsequent microbending loss. The resulting fiber does achieve fatigue resistance. However, long hermetic lengths have not passed time/temperature stress tests. A silicon oxynitride coating has passed stress tests, has exhibited no optical performance degradation, and provides substantial fatigue resistance. But, the silicon oxynitride coated fibers are not suitable for use under high stress in boiling water. Other artisans have found that the superior fatigue resistance of the silicon oxynitride-coated fiber allows significantly higher design stresses in service as compared to the polymer-coated fiber. Others have applied ion assisted deposition (IAD) techniques to deposit MgF.sub.2, SiO.sub.2 and Al.sub.2 O.sub.3 /SiO.sub.2 thin film structures on fluoride glass substrates at ambient substrate temperature of approximately 100 degrees C. The coatings deposited using IAD improve the environmental durability of the fluoride glass and appear to have reasonably good optical characteristics. Without application of IAD, the deposited coatings are not durable and have poor adhesion.